In a liquid crystal display device or a planar light emitting device, a fluorescent tube such as a cold cathode tube or a hot cathode tube bas been used as a light source. For example, in a liquid crystal display device such as a liquid crystal television or a liquid crystal monitor, a planar lighting device, i.e., a backlight unit (hereinafter, also referred to as BLU), is used for emitting an illumination light from a backside of a liquid crystal display panel to light the liquid crystal display panel.
At present, the light source portion of the liquid crystal display device, i.e., the backlight unit, adopts a method called a direct type which includes a liquid crystal display panel (light diffusion plate) disposed directly above a light source for lighting in most instances. For the light source of the BLU of this method, usually, a fluorescent tube such as a cold cathode tube or a hot cathode tube is used. For example, in this method, a plurality of fluorescent tubes which are light sources are arranged on the backside of the liquid crystal display panel to secure a uniform light-amount distribution and necessary brightness with the inside set as a white reflection plane.
However, in the BLU of the direct type, a direct light from the fluorescent tubes is converted into a uniform planar light source, and thus there is a problem in that a certain thickness for suppressing nonuniformity of brightness (brightness unevenness) needs to be secured, in other words, there is a limit to thinning thereof.
On the other hand, as a method for realizing a BLU thinner than the BLU of the direct method, there is known a light guide plate method (also referred to as sidelight type or method), in which a light from a light source is converted into a planar light by using a transparent resin flat plate called a light guide plate. As described above, the backlight unit of the sidelight method using the light guide plate is configured by a light source such as a fluorescent tube for lighting, a light guide plate for causing a light emitted from the light source to enter through its end plane, and diffusing and reflecting the light therein to emit a planar light from a light exit plane, and a component such as a prism sheet or a diffusion sheet for uniforming the light emitted from this light guide plate to illuminate the liquid crystal display panel. According to this method, a light is caused to enter through the lateral face (end plane) of the light guide plate, and guided in the light guide plate to emit a planar illumination light from a top or bottom surface which is larger than the lateral plane.
This method, in other words, the BLU of the sidelight method, is capable of being made thinner than the backlight unit of the direct method, because it has a function of emitting the light entering from the end plane while the light guide plate guides the light.
In the conventional BLU of the sidelight type, the light entering through its lateral end plane has to be scattered in a direction substantially orthogonal to an advancing direction to be emitted from the light exit plane. Thus, a light guide plate of a flat plate type or a tapered type (hereinafter, referred to as wedge shape) having a thickness reduced in the light advancing direction, a tandem light guide plate having wedge-shaped light guide plates sequentially combined, or a light guide plate of a bridge type in which thick portions of wedge-shaped light guide plates are combined together in other words, a sidelight type, is used.
Further, there is proposed a backlight unit of a conventional sidelight method that uses a wedge-shaped light guide plate containing scattering particles mixed in a transparent resin to scatter a light (e.g., refer to Patent Document 1).
In a planar light source device disclosed in Patent Document 1, a light emitted from a fluorescent lamp is caused to enter to a wedge-shaped light guide body containing scattering particles of different refractive indices and extremely small particle diameters uniformly scattered therein. The light that has advanced in the light guide body after the entrance is reflected by an inclined backside of the light guide body and a reflector disposed therein to be emitted from a light extraction plane of the light guide body. At the same time, the light advancing in the light guide body is emitted from the light extraction plane of the light guide body by scattering action of the scattering particles.
Recently, a light emitting diode (LED) has been used as a light source in place of the fluorescent tube. This is because the LED has a feature and an advantage of using no mercury that is essential to the fluorescent tube and a possibility of higher emission efficiency compared with the fluorescent tube. There is also proposed a light guide plate technology that uses the LED as a light source, i.e., a planar lighting device (backlight unit) technology (refer to Patent Documents 2 to 4).
Patent Document 2 discloses a lighting device which uses a light emitting diode (LED) as a point-like light source, disposed in an end plane of a flat-plate light guide plate. Patent Document 3 discloses a liquid crystal display device which includes light emitting elements such as LEDs for backlight light sources disposed in one of a pair of opposing transparent substrates forming a liquid crystal panel. Patent Document 4 discloses a backlight which includes a white LED disposed in the end of a light guide plate formed of a plurality of blocks.
Patent Document 1: JP 08-271739 A
Patent Document 2: JP 11-007014 A
Patent Document 3: JP 08-248420 A
Patent Document 4: JP 2001-092370 A